The invention relates to an electrode for a combination of supercapacitor and battery, where the electrode has an active structure. Further aspects of the invention concern processes for producing such an electrode and also a combined supercapacitor and battery cell comprising at least one such electrode.
Electric energy can, for example, be stored by means of batteries or by means of capacitors. Batteries store the electric energy in the form of chemical reaction energy and capacitors store the electric energy in the form of charges on capacitor electrodes. A battery comprises at least one positive electrode and at least one negative electrode which are separated by means of a separator. Owing to their high energy density, use is frequently made of lithium ion batteries which also display a low self discharge. In the lithium ion battery cells used in lithium ion batteries, lithium ions migrate from one electrode to the other electrode during charging and discharging of the battery. As active materials for the electrodes, use is frequently made of intercalation materials which are able to reversibly incorporate and release lithium ions. A lithium ion conductor is used for transport of the lithium ions. In the case of lithium ion battery cells used at present, which are employed, for example, in the consumer sector (cellular telephone, MP3 player, etc.) or as energy store in electric or hybrid vehicles, the lithium ion conductor is frequently a liquid electrolyte which, for example, contains the lithium electrolyte salt lithium hexafluorophosphate (LiPF6) as a solution in organic solvents.
In batteries, the specific power offtake, i.e. the discharge rate, also referred to as C rate, is limited. In addition, it is possible for the battery to be damaged when the battery is discharged at high C rates because the internal currents become too high. The same problem occurs during charging of a battery: charging of a battery can also be carried out only at a particular C rate without damaging the battery. The C rate is defined as the maximum permissible charging or discharging current divided by the capacity of the battery.
For the purposes of the present patent application, the term battery or battery cell is used as is customary in the conventional language, i.e. the term battery or battery cell encompasses both a primary battery or primary battery cell and also secondary batteries or secondary cells. The latter are also referred to as rechargeable batteries or rechargeable battery cells.
In contrast to batteries, capacitors can provide very high charging and discharging currents without being damaged. For this reason, capacitors are suitable as replacement for batteries or for supplementing batteries by means of a parallel connection of battery and capacitor when a high degree of reliability and high charging and discharging currents are required.
Capacitors comprise two electrodes which are arranged parallel to one another at a small spacing. A dielectric is generally present in-between. Various construction types of capacitors are known in the prior art, with supercapacitors having a specific capacitance which is up to 10 000 times as great as that of conventional electrolyte capacitors. However, a supercapacitor achieves only about 10% of the capacity of a battery of the same weight. In the case of supercapacitors, no dielectric is used between the two electrodes. The structure of these supercapacitors resembles that of a battery and comprises two electrodes which are mechanically separated from one another by a separator and are electrically connected to one another by means of an electrolyte. Application of a voltage to the capacitor results in formation of double layers on the electrodes, in which double layers a layer of positive charges and a layer of negative charges are formed in a mirror-like arrangement. In addition, pseudocapacitances can be formed when ions come into direct contact with the surface of the electrode and release an electron to the electrode. The total capacitance of the supercapacitor is then made up of the double layer capacitance and the pseudocapacitance.
A disadvantage of the prior art is that there is no simple component which combines both battery properties and capacitor properties.